The present invention relates to a metal cap which hermetically seals bottles, and is particularly aimed at glass bottles containing liquid foodstuffs under pressure.
This crown-cap is already well known and has been used for some time by bottling companies to seal many types of glass bottles, above all for drinks bottled under pressure, such as beer, Coca-Cola, lemonade, etc.
The circular central body of the cap has a suitable pressure-tight lining (usually made of synthetic, thermoplastic material for example PVC, polyethylene derivatives, or natural material such as cork) on the internal surface which is placed against a bottle neck opening. the outer crown is made up of corrugations which may be bent down to form a plurality of teeth which hold the cap firmly on to the rim of the bottleneck.
It is also well known that you need a bottle-opener always in hand to open a bottle sealed with the classical crown-cap.
This particular "problem" with opening has led some drinks manufacturers to produce a so-called "hybrid" solution: by closing the bottle with a screw-off crown cap. In practice this particular type of cap (in commercial jargon called "twist off") is made up of a normal crown-cap with twenty one teeth which is firmly placed around the rim of the bottle, which is obviously fitted with the necessary thread for unscrewing the cap. So the twist off cap has basically the same structure as the crown-cap but the teeth are fixed over the external thread which protrudes from the bottleneck.
The main problem with this solution is that it is not very comfortable to open by hand. The cap must be fix very firmly to the bottle to maintain the pressure of the liquid inside the bottle and to avoid the risk of leakage during normal handling. So a certain amount of force which not everyone may have, is needed to rotate the cap in order to open the bottle. this operation also carries the risk of injuring or cutting the hands due to the "sharp edges" of the teeth on the crown.
With this in mind several devices have been proposed to enable users to "twist off" the bottle cap swiftly (see also Canadian patent N. 1.252.431), but these are obviously considered accessories and must be on hand for the user to unscrew the cap.